In wireless communication systems, such as defined by 3GPP Long Term Evolution (LTE/LTE-A) specification, user equipments (UE) and base stations (eNodeB) communicate with each other by sending and receiving data carried in radio signals according to a predefined radio frame format. Typically, the radio frame format contains a sequence of radio frames, each radio frame having the same frame length with the same number of subframes. The subframes are configures to perform uplink (UL) transmission or downlink (DL) reception in different Duplexing methods. Time-division duplex (TDD) is the application of time-division multiplexing to separate transmitting and receiving radio signals. TDD has a strong advantage in the case where there is asymmetry of the uplink and downlink data rates. Seven different TDD configurations are provided in LTE/LTE-A systems to support different DL/UL traffic ratios for different frequency bands.
FIG. 1 (Prior Art) illustrates the TDD mode UL-DL configurations in an LTE/LTE-A system. Table 100 shows that each radio frame contains ten subframes, D indicates a DL subframe, U indicates an UL subframe, and S indicates a Special subframe/Switch point (SP). Each SP contains a DwPTS (Downlink pilot time slot), a GP (Guard Period), and an UpPTS (Uplink pilot time slot). DwPTS is used for normal downlink transmission and UpPTS is used for uplink channel sounding and random access. DwPTS and UpPTS are separated by GP, which is used for switching from DL to UL transmission. The length of GP needs to be large enough to allow the UE to switch to the timing advanced uplink transmission. These allocations can provide 40% to 90% DL subframes. Current UL-DL configuration is broadcasted in the system information block, i.e. SIB1. The semi-static allocation via SIB1, however, may or may not match the instantaneous traffic situation. Currently, the mechanism for adapting UL-DL allocation is based on the system information change procedure.
In 3GPP LTE Rel-11 and after and 4G LTE, the trend of the system design shows the requirements on more flexible configuration in the network system. Based on the system load, traffic type, traffic pattern and so on, the system can dynamically adjust its parameters to further utilize the radio resource and to save the energy. One example is the support of dynamic TDD configuration, where the TDD configuration in the system may dynamically change according to the DL-UL traffic ratio.
The Next Generation Mobile Network (NGMN) Board, has decided to focus the future NGMN activities on defining the end-to-end (E2E) requirements for 5G. Three main applications in 5G include enhanced Mobile Broadband (eMBB), Ultra-Low Latency services (ULL), and massive Machine-Type Communication (MTC) under milli-meter wave technology, small cell access, and unlicensed spectrum transmission. Specifically, the design requirements for 5G includes maximum cell size requirements and latency requirements. The maximum cell size is urban micro cell with inter-site distance (ISD)=500 meters, i.e. cell radius is 250˜300 meters. For eMBB, the E2E latency requirement is <=10 ms; for ULL, the E2E latency is <=1 ms. Furthermore, multiplexing of eMBB & ULL within a carrier should be supported, and TDD with flexible uplink and downlink (UL/DL) ratio is desirable.
Under the existing LTE TDD frame structure, which subframe can be UL or DL is fixed within a radio frame. As depicted in FIG. 1, the latency for HARQ-ACK is 4˜6 ms. Also, there is up to 9 ms latency between UL sounding and DL transmission. Even under dynamic TDD configuration, the TDD configuration can only change every 10 ms (one radio frame). Such latency performance obviously cannot meet the 5G requirements. A new flexible frame structure is sought to meet the 5G requirements.